There is known a damping force adjustable shock absorber including a cylinder, a piston slidably inserted into the cylinder, a piston rod movably inserted into the cylinder and connected to the piston, an expansion-side chamber and a contraction-side chamber partitioned by the piston inside the cylinder, an intermediate tube provided to envelop the cylinder to form a discharge passage in conjunction with the cylinder, an outer tube provided to envelop the intermediate tube to form a reservoir for storing hydraulic oil in conjunction with the intermediate tube, a charge passage that allows only a flow of hydraulic oil directed from the reservoir to the contraction-side chamber, a rectification passage provided in the piston to allow only a flow of hydraulic oil directed from the contraction-side chamber to the expansion-side chamber, and a damping force variable valve provided between the discharge passage and the reservoir.
In the shock absorber described above, the hydraulic oil flows from the cylinder to the reservoir through the discharge passage due to functions of the rectification passage and the charge passage in both the expanding and contracting motions. In addition, a damping force exerted by the shock absorber can be adjusted by controlling the resistance to the flow of hydraulic oil by using the damping force variable valve (for example, see JP 2009-222136 A).
In this manner, a damping force can be adjusted in the shock absorber described above. Therefore, it is possible to improve a vehicle ride quality by exert a damping force optimized to a vehicle vibration. In addition, in the shock absorber described above, the damping force variable valve is provided outside the cylinder. Therefore, it is very advantageous in that it is not required to sacrifice a stroke length of the shock absorber and harm loadability of a vehicle, compared to other types of shock absorbers in which the damping force variable valve is provided in the piston.